A sandbar is exposed by low water on Portage Lake in Onekama, Mich., last November, which has made nearby docks and marinas largely unusable. Water levels in Lake Huron and Lake Michigan are the lowest they’ve ever been since record keeping began in 1918, according to the US Army Corps of Engineers.

Chicago — Water levels in Lake Huron and Lake Michigan are the lowest they’ve ever been since record keeping began in 1918, according to the US Army Corps of Engineers. That finding, which corresponds to a decade of diminished precipitation and higher temperatures, worries barging and commercial fishing interests, with both industries saying it will have harsh economic consequences.

The Corps reported this week that both lakes were 29 inches below their long-term average and that levels had declined 17 inches since January 2012 alone. The agency said the water levels have been below average for the past 14 years, which is the longest period of sustained below-average levels since 1918. Continued record lows are expected for the first few months of this year.

A hot and dry summer in the Midwest increased evaporation throughout the fall, followed by several lower-than-average snowfalls in the winter. That led to a seasonal rise in both lakes last year of only four inches, which is eight inches below normal, the Corps said.

The decline presents a long-term threat to carriers that rely on the Great Lakes system for transport. The American Waterways Operators, an advocacy group for the tugboat, towboat, and barge industry in Arlington, Va., said that every inch of water loss in the Great Lakes decreases the carrying capacity of a single barge by 17 tons of cargo. That means that the loss of a foot would cause a capacity loss of 204 tons per barge.

In a statement, the organization called the water loss “a severe, ongoing situation.”

Similarly, the Lake Carriers’ Association, an advocacy group representing 17 companies that use the Great Lakes for cargo transport, estimated that some 10,000 tons of cargo could not be transported in 2012. Charter boat and commercial fishing operators around the Great Lakes region also said they fear tourism dollars will decline during their busy summer and fall season.

Researchers blame excessive dredging, both by the Chicago River system, which diverts water to the South, and the St. Clair River, which diverts water to deepen navigational channels to Lake Erie, Lake Ontario, and eventually the Atlantic Ocean. The International Joint Commission, a Canadian-US organization that looks for solutions to waterway issues, said it will release a report next month that examines the effect of dredging, among other factors, on water levels in the Great Lakes.

David Allan, a professor in the School of Natural Resources and the Environment at the University of Michigan in Ann Arbor, says that, while the water drop is indeed historic, data shows that it follows a somewhat cyclical pattern. The current level, for instance, is just short of previous low points in the 1930s and '60s.

Whether or not water levels will continue to drop is uncertain, he says, as is whether or not climate change is entirely responsible. He says that factors such as warming temperatures and decreased precipitation are present, but suggests that others could be at play as well, such as dredging and natural fluctuations in water levels.

“The science is maybe a little early to fully understand fluctuations in order to fully identify climate change as a driver of this drop, but there is evidence of climate change over the last 30 or 40 years that would lead you to expect these water level drops to occur,” Dr. Allan says.

Because not enough is known to predict where water levels will go from here, the Corps has not yet taken action to stop them from dropping. Mr. Kompoltowicz of the Corps told AP that funding was limited and that nothing would be considered until 2015. Allan says it is unlikely the Corps will launch an expensive and long-term project without the certainty of a forecast.

“You could put in water-control structures at the St. Clair and hold back more water for Michigan and Huron, but there are downstream consequences if the water level drop is a natural one and water gets back up four or five feet higher than it is now,” Allan said. “So if you can’t conclude water levels are going to be low forever, you’re not going to put in an expensive engineering project that would take a very long time.”